Microstrip technology in general is well known in the art. Microstrip lines are generally perceived as a form of transmission line. They typically provide a useful means of signal transmission for short-distance applications. Moreover, microstrip configurations are commonly utilized as a basic building block in hybrid microwave microcircuits. The transmission line optimally provides matched interconnections between signal sources and loads.
FIG. 1 illustrates a perspective view of a typical prior art microstrip configuration designated generally at 10. Configuration 10 is typically a planar structure including a substrate 12. Substrate 12 may be of any known dielectric material such as alumina or polyamide. The height of substrate 12 varies, and may be on the order of 0.010 to 0.062 inches. Both the thickness and type of material of substrate 12 are commonly chosen based on other circuit components (not shown) which it may support, as well as the material thermal conductivity, surface finish, dielectric constant, thermal coefficient of expansion and ability to withstand processing temperatures.
Substrate 12 supports a microstrip conductor 14 which is formed by known techniques. For example, microstrip conductor 14 may be deposited by thin-film techniques using deposition in a vacuum by electron-beam evaporation or sputtering. Microstrip conductor 14 is commonly on the order of 0.010 to 0.125 inches in width and 0.0010 to 0.0023 inches in height. The length of microstrip conductor 14 is established to accommodate the location of the source and destination (not shown) for the signal transmitted by the conductor. A ground plane 16 is deposited along a second surface of substrate 12. Ground plane 16 is a suitable width relative to that of conductor 14 such that it approximates an infinite plane.
It should be noted that FIG. 1 illustrates configuration 10 standing alone only by way of example. Typically, such a configuration is included in a much larger structure, such as a board to be used in the assembly of a contemporary computer system (e.g., a motherboard). Such a board may be fabricated to include a portion having a dielectric layer that separates a ground plane from a microstrip conductor. In systems applications, these boards often implement, or are exposed to, potentially strong electromagnetic signals. For example, the microstrip conductor may itself transmit high frequency signals. Thus, a need arises to prevent these signals from interfering with other circuitry in close proximity to the microstrip conductor. Conversely, the circuitry proximate the microstrip conductor may also involve high levels of electromagnetic interference. Naturally, it is likewise desirable to shield the microstrip conductor from potential interference created by such other circuitry.
It is therefore an object of the present invention to provide an electromagnetically shielded microstrip circuit and a method of fabricating such a circuit.
It is a further object of the present invention to provide such a circuit having minimum refections or distortions of the electric field surrounding the microstrip conductor.
It is a further object of the present invention to provide such a circuit assembled at least in part by an automated assembly machine operable to select from a plurality of components.
It is a further object of the present invention to provide such a circuit having a low impedance device selected from automated assembly machine components, wherein the combination of the low impedance device coupled to the ground plane forms a circumferential shield around the strip conductor.
Still other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.